The present invention relates to an opto-electronic device and fabrication method thereof, more particularly to the alignment of a patterned ohmic contact layer in an opto-electronic device.
The ohmic contact layer in question interfaces between an electrode and a diffusion area on the surface of a semiconductor substrate, to ensure an ohmic contact between the electrode and the diffusion area. If the opto-electronic device is a light-emitting diode (LED), for example, the diffusion area is an area from which light is omitted when the electrode supplies driving current. If the device is an LED array, there are a plurality of such areas, with respective electrodes.
Especially in the case of an LED array, the ohmic contact pattern needs to be accurately aligned with the diffusion areas. This is difficult, because the diffusion areas are conventionally defined by a diffusion mask, and the ohmic contact layer is patterned by an etching mask. The two masks are formed in different steps in the fabrication process, and their mutual alignment is almost never perfect. Mask alignment error, and the resultant misalignment of the ohmic contact pattern with the diffusion pattern, can cause such problems as inadequate electrical contact between electrodes and diffusion areas, and short circuits between electrodes and non-diffusion areas. These problems are particularly acute in high-density arrays, where they significantly reduce manufacturing yields and degrade light-emitting characteristics.
LEDs and LED arrays are used as light sources for many purposes. To name one application, LED arrays are used as light sources in electrophotographic printers. The above-mentioned mentioned problems present an obstacle to the development of high-quality LED printers with high dot resolution.
An object of the present invention is to improve the manufacturing yields of opto-electronic devices having an ohmic contact layer.
Another object of the invention is to manufacture opto-electronic devices with uniform characteristics.
The invented method of fabricating an opto-electronic device comprises the steps of:
(a) forming an ohmic contact layer on the surface of a semiconductor substrate of a first conductive type;
(b) forming a diffusion source layer on the ohmic contact layer;
(c) forming a mask on the diffusion source layer;
(d) etching the diffusion source layer to form a pattern defined by the mask;
(e) diffusing impurity atoms from the diffusion source layer into the ohmic contact layer and the semiconductor substrate, forming a diffusion area of a second conductive type in the semiconductor substrate;
(f) etching the ohmic contact layer, using either the above mask or the diffusion source layer as an etching mask; and
(g) forming an electrode making contact with the ohmic contact layer.
The diffusion step (e) may be performed either before or after the step (f) of etching the ohmic contact layer. When step (e) is performed before step (f), the diffusion source layer may be removed after step (f), and an isolation film deposited to isolate the electrode from the substrate. When step (e) is performed after step (f), a dielectric film, deposited between steps (e) and (f), may be used both as a diffusion cap film and as an inter-layer isolation film isolating the electrode from the substrate. An additional isolation film may be deposited on the dielectric film to improve the isolation.
The invention also provides opto-electronic devices fabricated by the method above.
In the invented fabrication method, since the ohmic contact layer is self-aligned with the diffusion source layer, it is also self-aligned with the diffusion area. Manufacturing yields are improved and uniform device characteristics are obtained because the ohmic contact layer is always aligned correctly with the diffusion area.